1. Field of the Invention
The present invention relates to a system for drilling holes in hard material, such as rock.
2. Description of the Related Art
Conventional drilling systems employ roller cone bits which operate by successively crushing rock at the base of a bore. Roller cone bits are disadvantageous because rock is typically very resistant to crushing. Other known rock drilling systems employ drag bits. Conventional drag bits operate by shearing rock off at the base of the bore. Drag bits can be more efficient than roller cone bits because rock is typically less resistant to shearing than to crushing.
However, neither roller cone bits nor conventional drag bits attack rock in a way that exploits its weakest property, tensile strength. In addition, roller cone bits and conventional drag bits require the application of substantial axial force to be effective. Consequently, the known systems require drilling equipment of substantial size and power. Likewise, the known systems generally require the use of drill collars, and make it difficult to use stabilizers. Developing sufficient axial thrust is particularly difficult when using short drill strings or when drilling horizontally. Conversely, with long drill strings, the axial force due to the weight of the drill pipe itself may exceed the efficient operating parameters for a particular drill bit, requiring a careful balancing of forces by exerting an upward force on the drill string at the surface.
Prior art drill bits and other excavating devices are disclosed in U.S. Pat. No. 187,705 (Collins), U.S. Pat. No. 2,250,670 (Joy), U.S. Pat. No. 2,794,623 (Stokes), U.S. Pat. No. 3,043,383 (Newbold), U.S. Pat. No. 3,094,178 (Newbold), U.S. Pat. No. 3,174,801 (Owen), U.S. Pat. No. 3,749,189 (Boehm), U.S. Pat. No. 4,275,796 (Kleine), U.S. Pat. No. 4,299,295 (Gossard), U.S. Pat. No. 4,087,131 (Peterson) and U.S. Pat. No. 4,932,482 (DeLucia).